1. Field of the Invention
This invention relates to methods of and apparatus for edge-forming metallic plates and more particularly but not exclusively relates to the edge-forming of lithographic plates, and metal-backed letter press plates.
2. Description of the Prior Art
In the preparation of lithographic plates for printing usually two opposite edges of the plate are bent-over for insertion in the wells of a printing drum of a rotary offset lithographic printing machine, where the bent-over ends are engaged by clamping means to secure the plate to the drum. In such cases it is necessary that the edge forming operation provides a plate having accurately formed edges, which is not deformed and/or stressed and which is not abraded and in which the formed edges are in precise relationship to the image. In one known edge forming machine in which metal beams are used to form the edges of the plate, about a metal former, such precision in formed edge/image relationship cannot be ensured. This is because the metal beams are subjected to slight bending during edge-forming, and the machine to wear of its parts such as the metal beams and bearings and former thereby increasing play with time and producing inconsistent results. If for example it is necessary to replace one of a plurality of lithographic plates which print together on the same printing cylinder, after some time has elapsed, then because of wear and deformation of the machine the edge-formed edges of the fresh plate usually have a different formed edge/image relationship to the replaced plate and to the other plates, resulting in out-of-register printing. Moreover, any inaccuracies, deformations or stresses in a particular lithographic plate caused during edge-forming will result in displacement of the photographic image when the plate is secured to the printing drum and thus also in out-of-register printing. Furthermore, any abrasion of the plate can remove the photographic emulsion from the printing surface of the plate. Thus, it is not possible with such apparatus to ensure that for large numbers of lithographic plates the formed edge/image relationship will be precise for each particular plate and the same for every plate.
Accordingly it is an object of this invention to provide a method of and apparatus for edge-forming metallic plates, in which the aforesaid disadvantages are minimized or eliminated.